Finally built my trailer with 20" wheels:


1" square tubing, all bolted on, no welds.


With my DL-1.


Wheel attachment.


Towbar attachment & tension cable, rope loops for bungee cords.


Tension cable (rear).


Towbar.


Hitch.


Hitch attachment.

Probably a bit overbuilt, fairly heavy at 17kg, but easy to tow, even on a dirt road with mud patches. Max load 25 kg so far, but I think it could easily carry 50kg.
Thanks to UC forum members for all the useful info!